An NMR flow meter can measure the flow of materials using NMR procedures provided most of the variables are fixed. A flow meter of such construction typically has a fixed sensitive region. That refers to the region which is exposed to the requisite magnetic field, that field being indicated by the symbol H.sub.o. The device is not able to distinguish between changes in fill factor within the sensitive region or, non-homogeneous variations in composition of the flowing material.
This apparatus works quite well so long as one of the two important factors (non-homogeneous material composition and fill factor) is held constant, or only one is permitted to vary. As will be understood the response of the NMR detector system is based on population of a particular nuclei species within the sensing region. That population can be reduced either by incomplete filling or a non-homogeneous variation in the material. Generally, measuring devices can be used with pipes or conduits which are sized to enable complete filling. Sometimes, the application of the measuring device will not permit such a narrow constriction in the flow path of the material undergoing measurement.
This disclosure sets forth an auxiliary magnetic resonance detector. It can either be a nuclear magnetic resonance detector or an electron spin resonance detector (known as NMR and ESR). The auxiliary detector utilizes a small coil which defines a relatively small sensing region for the auxiliary detector. This small coil is usually positioned in the bottom portions of the conduit to assure that it is filled at all times. Thus, it is provided with 100% filling within its particular sensing region, and filling of this volume assures that the fill factor is meaningless to the signal provided by the auxiliary magnetic resonance detector. Thus, it is sensitive only to variations in homogeneity of the flowing material. The auxiliary detector therefore forms an output signal which may be described as a calibration factor. It is a calibration factor relating to the homogeneous material or absence of homogeneity in the flowing material.
The flow volume can then be determined from this data. Briefly, the flow volume is dependent on the two measurements from the two detector systems and constants which takes into account system parameters, proportionality, linear operation and the like. The actual flow volume can therefore be defined as being the function of the two output signals or is given by the relationship f(x,y) where the two variables are the output signals. This accomodates changes in composition and fill factor within the conduit.
One reference of note is Handel, U.S. Pat. No. 2,948,845. This disclosure is directed to a system which does not involve flowing materials. In Aske, U.S. Pat. No. 3,379,979 a system involving duel channel operation (see FIG. 3 thereof) is set forth. It does not concern measuring volume flowing in a pipe or conduit. In Auld, U.S. Pat. No. 4,286,216 a metal flaw detector is set forth. It works with disturbances in the field. Again, it is not particularly concerned with a flow system. In Abe, U.S. Pat. No. 3,932,805 magnetic field superimposition is described to obtain a controllable gradient in the field and hence a resonance frequency in the output data. This reference does not cooperate with a variable fill factor flow system.
In summary, this disclosure sets forth both a method and apparatus for making flow measurements in a situation involving partial filling of the flow path with materials which vary in composition as to the sensitive material detected by the flow measuring apparatus.